a. Field of the Invention
The present invention relates to an electronic image capture system for capturing an electronic image having reduced noise, and a method for capturing an electronic image having reduced noise, for example from a digital camera device.
b. Related Art
There is a demand for and a trend towards miniaturisation of many types of image capture device. For example, digital cameras may be integrated in cell phones, which are ever more compact. Such a camera will need to have a small diameter lens compared with a lens in a more conventional dedicated hand-held digital camera. For this reason, and also for reasons of reduced cost, the detector arrays used in cell phones and other miniaturised digital cameras have become smaller. At the same time, users expect a larger number of image capture pixel elements for increased image definition, with the result that the size of the pixel elements is greatly reduced.
Smaller solid state pixel elements, for example of the CCD or CMOS type, have a decreased capacity for holding electric charge. Although the inherent efficiency of a small pixel element may be comparable to that of a larger element, a reduced detector “well size” will limit the total number of photons each pixel element can sense. The detector shot noise varies with the square root of the captured image intensity, with the best signal to noise ratio normally being in regions having the greatest image intensity. Thus, smaller arrays having smaller pixel elements, and lower maximum signals, are usually noisier than detector arrays having larger size pixel elements. Other more constant sources of noise which do not depend on image intensity, particularly readout noise, will add to the shot noise, and will normally contribute a larger proportion of the total signal in an array with lower maximum signal.
As a result, there is an increased need for better noise reduction in many electronic image capture devices, without unduly adding to the hardware cost of such devices or worsening other image capture performance metrics, for example the maximum number of frames per second, or the lifetime of portable power sources.
A further problem arises in that the increased noise due to shrinking pixels will limit the use of tone-mapping to improve visual image quality, since tone mapping requires gain to be applied in some parts of the signal range and this will make noise even more visible.
The relative strength of noise in an electronic image will normally be highest in dimly lit areas of a scene, where the number of photons captured will be lowest for a given exposure time, aperture restriction or electronic gain setting (for convenience, referred to hereinafter collectively and individually simply as “exposure”). The exposure will need to be such that the brightest parts of the image are within the saturation limits of the pixel elements, with the result that dimly lit areas at the same exposure may be well below saturation, and have a correspondingly lower signal-to-noise ratio (S/N).
However, from a perceptual point of view, noise in dimly lit portions of a scene may be less visible, or less important, than noise in brighter portions if the dim areas are not of interest to a viewer of the image. In considering the importance of noise reduction in an electronic image, it is therefore not sufficient to consider only the effect of noise in dimly lit areas of a scene.
It is an object of the invention to provide a more convenient apparatus and method for capturing an electronic image with reduced visible noise.